The pertinent prior art known to the inventor is U.S. Pat. No. 4,136,978 issued Jan. 30, 1979. The printer head of said patent includes 7 to 9 modules each controlling a stylus for printing 7 to 9 level dot matrix characters. Each module includes a printer arm carrying a stylus at one end, the arm being supported by a prestressed crossed flexure arrangement at the other end. A soft iron insert which forms the armature of an electromagnet is carried by the arm adjacent its supported end. The electromagnet and a permanent magnet together with the armature are included in a magnetic circuit with the electromagnet bucking the permanent magnet when energized.
The electromagnet is normally de-energized and the insert is attracted to the pole of the electromagnet by action of the permanent magnet. Energization of the electromagnet reduces the flux in the magnetic path, the armature is released and the prestressed crossed flexures move the arm so that the stylus impacts the surface on which printing is to occur. The stylus arm rebounds from said surface and is captured by the pole of the electromagnet; the latter having been de-energized by the time the rebounding arm approaches its nonprinting position.
The aforesaid modules have been well received in the market place, providing printing speeds equal to low speed line printers (600 characters per second) at a cost well below the cost of such line printers. Some problems, however, have become apparent as a result of extended use of the device. Lack of rigidity between the pole face and crossed flexure mount result in fidgeting of contacting armature and pole surfaces with resultant wear. In order to achieve the desired life of the head, upwards of 500 million characters, it has been necessary to oil the abutting surfaces.
Oiling of the abutting surfaces introduced two problems the first of which is that the operators of the printer do not want to oil the heads even though such procedure requires applying only 8 drops of oil about once every week or two. The second problem is that the oil causes the printer to occasionally drop a dot or two in the first character upon start up after the machine has been shut down; usually overnight.
In addition to the above problems the prior head is somewhat difficult to assemble and is more costly than desired.
The head of the patent is saleable and is currently being sold. The problems described, although being somewhat bothersome, do not prevent sale of the apparatus because its speed of 600 characters per second at a reasonable cost provides a ready market for the device.